A current control device maintains a given magnitude of current along a particular current path, e.g., a series combination of circuit components. Conventional current control devices typically require either a minimum potential across terminal leads, or react undesirably to slight variation in circuit parameters, e.g., deviation from expected power supply voltage or expected component characteristics.
One use of a current control device is driving a light emitting diode (LED), a common display device for electronic products. The brightness of an LED is a function of the amount of current passing through the LED. To control the brightness of an LED then, it is sufficient to control the magnitude of current passing through the LED. To provide consistent LED brightness, a consistent magnitude of current must pass through the LED.
A number of LED display devices connected in series have a desired level of brightness by controlling the amount of current passing through the series combination. As a diode, however, the voltage drop across an LED is substantially independent of the current it carries. Much of the potential across the LED series combination, therefore, can be taken by voltage drops across the LED display devices. As a result, less voltage potential remains across the current control device, and its operation may be impaired if this remaining potential is insufficient.
This is particularly critical when, for example, a relatively small supply voltage is used to drive a series combination of LED display devices. A conventional current mirror placed in series with an LED provides current control substantially independent of the voltage drop, i.e., forward voltage, of the LED. However, a simple current mirror, for example, an n-channel MOS-G device, typically requires at least 2 volts across its drain and source terminals for proper operation. For a 5 volt supply voltage and a pair of series coupled LED display devices, each having a 2 volt forward biased voltage, only 1 volt remains across the current mirror for current control, and the device cannot operate as desired. It is, therefore, desirable that a current control device operate with a small potential across its terminal leads.
A second current control approach uses an output transistor in its linear mode with a resistor circuit for setting current flow through the transistor. While this approach is less sensitive to the potential across the transistor, it is quite sensitive to variation in the supply voltage, LED forward voltage, and absolute resistor values obtained. A slight variation in these circuit parameters can result in significant variation in LED brightness.
Some applications require an array of adjacent LED devices. Each LED of the array is desirably of substantially identical brightness when activated. For example, if the LED array is part of a seven segment display, it is desirable that each segment of the display appear with matching brightness. Also, some laser printers use an array of hundreds of LED light sources, and the quality of printed output obtained depends on consistency of LED brightness. To accomplish consistent LED brightness, currents of substantially matching magnitude must pass through each LED.